Display device

ABSTRACT

The inventors found out that in the case of performing a low gray scale display in which a very small amount of current is supplied to a light emitting element, variations in threshold voltages of driving transistors become notable since the gate-source voltage is low. In view of this, the invention provides a display device in which variations in the threshold voltages of the driving transistors are reduced even in the low gray scale display, and a driving method thereof. According to the invention, a gate-source voltage of the driving transistor is set higher in the low gray scale display than that in the high gray scale display. As one mode to achieve this, different power source lines are provided for the low gray scale display and the high gray scale display and their potentials are set to be different.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device having a self-lightemitting element.

2. Description of the Related Art

In recent years, a display device using a light emitting element(self-light emitting element) has been actively researched anddeveloped. Such a display device is widely used as a display of aportable phone or a monitor of a computer by taking advantages of highimage quality, thin design, lightweight and the like. In particular,such a display device provides high response speed suitable fordisplaying moving images, low voltage, low power consumption drive andthe like, therefore, its wide applications such as a new generationportable phone and a portable information terminal (PDA) are expected.

A light emitting element is also referred to as an organic lightemitting diode (OLED) and has a structure having an anode, a cathode,and a layer containing an organic compound between the anode and thecathode. The amount of current supplied to the light emitting elementand a luminescence of the light emitting element are in a certainrelationship. The light emitting element emits light at a luminescenceaccording to the amount of current supplied to the layer containing theorganic compound.

The major methods for displaying a multi-gray scale image by a lightemitting device using a light emitting element are a voltage inputmethod and a current input method. According to the voltage inputmethod, a video signal inputted to a pixel is inputted to a drivingelement, thereby a luminescence of the light emitting element iscontrolled. According to the current input method, a set signal currentis supplied to a light emitting element, thereby a luminescence of thelight emitting element is controlled. Both methods can employ an analogdriving method (analog gray scale method) and a digital driving method(digital gray scale method).

In order to prevent variations in characteristics of element drivingthin film transistors which correspond to driving elements of lightemitting elements in the voltage input method, a semiconductor deviceprovided with a compensating thin film transistor between a drivingpower source and the element driving thin film transistor has beensuggested.

[Patent Document 1]

Japanese Patent Laid-Open No. 2002-175029

In the aforementioned patent document, variations in the thresholdvoltage of transistors (referred to as driving transistors) whichcontrol driving current according to gray scales is not taken intoconsideration. However, the inventors figured out that in the case ofperforming a low gray scale display in which a minute current issupplied to the light emitting element, variations in threshold voltages(Vth) become notable since a gate-source voltage (Vgs) of the drivingtransistor, that is a potential difference between the gate electrodeand the source electrode is small.

SUMMARY OF THE INVENTION

The invention provides a display device in which variations in thresholdvoltages (Vth) of driving transistors are reduced even in a low grayscale display, and a driving method thereof.

In view of the aforementioned, according to the invention, a gate-sourcevoltage (Vgs) of a driving transistor is raised higher than theconventional one in the low gray scale display. As a result, effect ofthe variations in the threshold voltage (Vth) of driving transistors canbe less even in the low gray scale display. As one mode of theinvention, different power source lines are provided for the low grayscale display and a high gray scale display, of which potentials aredetermined to be different. By setting the potential of the power sourceline for the low gray scale display higher than that of the power sourceline for the high gray scale display, a gate-source voltage (Vgs) of adriving transistor can be set high.

In this case, a light emitting period (display period) is controlled forobtaining a predetermined low gray scale display. As one mode of theinvention, the driving transistor and the power source line aredisconnected. As one mode of such a pixel configuration, a switch isprovided so that a current from the power source line is not supplied tothe light emitting element.

According to one mode of the display device of the invention, a signalline to which an analog signal is inputted, a first transistor and asecond transistor for driving a light emitting element, and a switch fordisconnecting the second transistor and a power source line areprovided. By using the first transistor based on the analog signal, acurrent is supplied from a first power source line to the light emittingelement. By using the second transistor based on the analog signal, acurrent is supplied from a second power source line to the lightemitting element, thus the switch is turned OFF per predeterminedperiod, which is after the predetermined period passes. In other words,the second transistor and the light emitting element are disconnected byusing the switch.

According to the invention, effect of variations in threshold voltages(Vth) of driving transistors can be suppressed in performing the lowgray scale display. As a result, luminance variations of light emittingelements can be reduced.

By controlling the light emitting period using the switch in performingthe low gray scale display, increase in luminance due to highgate-source voltage (Vgs) can be suppressed, thus a predeterminedluminance can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a pixel circuit of the invention.

FIG. 2 is a diagram showing a pixel circuit of the invention.

FIG. 3 is a diagram showing a pixel circuit of the invention.

FIG. 4 is a diagram showing a pixel circuit of the invention.

FIG. 5 is a diagram showing a pixel circuit of the invention.

FIG. 6 is a diagram showing a pixel circuit of the invention.

FIGS. 7A to 7C are diagrams showing a pixel configuration of theinvention.

FIGS. 8A to 8E are views showing electronic apparatuses each having apixel circuit of the invention.

FIG. 9 is a graph showing variations in currents flowing through drivingtransistors relative to gray scale.

FIG. 10 is a timing chart of the invention.

FIG. 11 is a timing chart of the invention.

FIGS. 12A and 12B are timing charts of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention will be fully described by way of example withreference to the accompanying drawings, it is to be understood thatvarious changes and modifications will be apparent to those skilled inthe art. Therefore, unless such changes and modifications depart fromthe scope of the invention, they should be construed as being includedtherein. Note that identical portions in embodiment modes are denoted bythe same reference numerals and detailed descriptions thereof areomitted.

In the following description, a transistor includes three electrodes: agate, a source, and a drain, however, the source electrode and the drainelectrode in particular cannot be clearly distinguished because of thestructure of the transistor. Therefore, when describing connectionsbetween the elements, one of the source electrode and the drainelectrode is referred to as a first electrode while the other isreferred to as a second electrode.

Embodiment Mode 1

In this embodiment mode, a pixel including a driving transistor for thehigh gray scale display and a driving transistor for the low gray scaledisplay is described.

First, a pixel configuration is described with reference to FIG. 1. Thepixel configuration of this embodiment mode includes a signal line 10 towhich an analog signal is inputted, a write switch (SW11), a firstdriving transistor 12 for the high gray scale display, a second drivingtransistor 13 for the low gray scale display, and a light emittingelement 14. The first driving transistor 12 and the second drivingtransistor 13 each has a function to drive the light emitting element14. The first driving transistor 12 and the second driving transistor 13are electrically connected to the light emitting element 14. A firstpower source line 16 is connected to the first driving transistor whilea second power source line 17 is connected to the second drivingtransistor through a control switch (SW20). For example, the first powersource line 16 functions as an anode line for a high gray scale displaywhile the second power source line 17 functions as an anode line for alow gray scale display.

A capacitor line 19 is connected to the write switch 11 through acapacitor (Cs18) which is not necessarily provided. In other words, whenthe first and second driving transistors each has a large gatecapacitance and a leak current from each transistor is an acceptableamount, the capacitor is not required to be provided. For example, whenthe first power source line or the second power source line has aconstant potential, one of them can be used as a capacitor line.

In such a pixel configuration, a signal line for the high gray scaledisplay and a signal line for the low gray scale display are notrequired to be provided, thus reduction in the aperture ratio can beprevented.

Next, an operation thereof is described.

The signal line 10 is inputted with a video signal as an analog signal,that is an analog potential (hereinafter this signal is referred to as avideo signal potential). The video signal potential for the high grayscale display is determined so that (a video signal potential for thehigh gray scale display)=(a potential of an anode line for the high grayscale display−|Vth| of the first driving transistor) is satisfied. Whenthe video signal potential for the high gray scale display is determinedlike this, a predetermined current is supplied to the light emittingelement from the anode line for the high gray scale display. Although avery small amount of current may be supplied from the anode line for thelow gray scale display, it can be ignored in many cases.

The video signal potential for the low gray scale display is determinedso that (a potential of the anode line for the high gray scaledisplay−|Vth| of the first driving transistor)<(a video signal potentialfor the low gray scale display)<(a potential of the anode line for thelow gray scale display−|Vth| of the second driving transistor) issatisfied. When the video signal potential for the low gray scaledisplay is determined like this, a predetermined current is supplied tothe light emitting element from the anode line for the low gray scaledisplay in the case of the low gray scale display. On the other hand, acurrent is not supplied to the light emitting element from the anodeline for the high gray scale display in the high gray scale display ofthis state.

Based on such a video signal potential, a current is supplied to thelight emitting element 14 from the first driving transistor 12 in thehigh gray scale display. Specifically, when the write switch 11 isturned ON, a charge is accumulated in the capacitor 18. After that, whena gate-source voltage (|Vgs|) of the first driving transistor 12 becomeshigher than the threshold voltage (|Vth|), the first driving transistor12 is turned ON. Then, a current according to a predetermined luminanceis supplied from the first power source line. As described above,depending on the video signal voltage and a voltage of the anode line, avery small amount of current is supplied from the anode line for the lowgray scale display as well, however, it can be ignored in many cases.The video signal voltage and the voltage of the anode line can bedetermined in consideration of the very small amount of current as well.

In the low gray scale display, a current is supplied from the seconddriving transistor 13 to the light emitting element 14. Specifically,when the write switch 11 is turned ON, a charge is accumulated in thecapacitor 18 similarly to the high gray scale display. After that, whena gate-source voltage (|Vgs|) of the second driving transistor 13becomes higher than the threshold voltage (|Vth|), the second drivingtransistor 13 is turned ON. Then, a current according to a predeterminedluminance is supplied from the second power source line. In this manner,the light emitting element 14 emits light. Even when the write switch 11is turned OFF, the light emitting element can keep emitting light byusing the charge accumulated in the capacitor.

In the low gray scale display, the light emission period can beshortened by using the control switch 20. Specifically, after apredetermined period passes, the control switch 20 is turned OFF. As aresult, |Vgs| of the driving transistor 13 for the low gray scaledisplay can be set higher, thus an effect of variations in the thresholdvoltage can be reduced. In the case of shortening the light emissionperiod by ⅕, for example, |Vgs| high enough to supply a current fivetimes as much can be applied to the second driving transistor 13. Then,the control switch 20 is turned OFF and the light emission period isshortened so that a predetermined luminance can be obtained. It is to benoted that the control switch 20 can operate by a line sequentialscanning at the same frequency as the write switch 11.

In this embodiment mode, switches such as the write switch 11 and thecontrol switch 20 can be formed of an analog switch and the like. FIG. 4shows a pixel circuit in which a thin film transistor (TFT) containingpolycrystalline silicon is used for the switches and each transistor ofa pixel.

A transistor 40 corresponding to the write switch 11 (also referred toas a switching transistor) and a transistor 41 corresponding to thecontrol switch 20 are provided. The transistor 40 is formed of ann-channel TFT while the transistor 41 is formed of a p-channel TFT It isto be noted that the transistor 40 may have a multi-gate structure inwhich a plurality of gate electrodes are provided for one semiconductorfilm, for example a double-gate structure in which two gate electrodesare provided. A gate of the transistor 40 is connected to a first scanline 42 while a gate of the transistor 41 is connected to a second scanline 43. The transistors 12 and 13 can be formed of p-channel TFTs.

It is to be noted that an operating region of a TFT can be divided intoa linear region and a saturation region. It is preferable that thetransistors 40 and 41 that function as switches operate in the linearregion while the transistors 12 and 13 for driving the light emittingelement 14 operate in the saturation region.

For the transistor, an amorphous silicon thin film transistor or otherthin film transistors can be used as well as a polycrystalline siliconthin film transistor. In other words, the structure of a transistor isnot limited in this embodiment mode.

In such a pixel, a timing at which the transistor 41 corresponding tothe control switch is turned ON or OFF is determined. The control switchand the write switch can operate at the same driving frequency and theymay also operate by a line sequential scanning. Accordingly, thedriving, that is the operation of the pixel in this embodiment mode canbe simplified.

FIG. 10 is a timing chart of the transistors 40 and 41. A High signal ora Low signal is inputted to the transistors 40 and 41 in one frameperiod. In this embodiment mode, a Low signal is inputted to thetransistor 41 in 30% of one frame period. Then, the light emissionperiod in the low gray scale display can be 30%. By setting the periodin which a Low signal is inputted to the transistor 41 to be as long asan integral multiple of the period in which a High signal is inputted tothe transistor 40, they can operate at the same driving frequency.

The timing at which a High signal or a Low signal is inputted to theseswitches is not limited to FIG. 10. For example, the transistors 40 and41 may be turned ON at the same time. This is because drivers for thetransistors 40 and 41 are provided respectively. The period in which aLow signal is inputted to the transistor 41 is not limited to FIG. 10.In the case of performing the low gray scale display, for example, aperiod in which a Low signal is inputted to the transistor 41 can bedetermined according to the gate-source voltage (|Vgs|) of the seconddriving transistor 13.

By turning ON and OFF the transistor 41, a predetermined luminance canbe obtained in the low gray scale display even in the case of increasingthe gate-source voltage (|Vgs|) of the second driving transistor 13.

Heretofore described is the case of dividing the gray scales into thehigh gray scale display and the low gray scale display, however, theymay be divided into three or more groups as well. For example, byproviding first to third power source lines as anode lines for the highgray scale display, the middle gray scale display, and the low grayscale display and providing driving transistors connected to each ofthem, the gray scales can be displayed as the high gray scale display,the middle gray scale display, and the low gray scale display.

According to this embodiment mode, the gate-source voltage (|Vgs|) ofthe driving transistor can be set higher in the low gray scale display,therefore, an effect of variations can be reduced. Moreover, a signalline for the high gray scale display and a signal line for the low grayscale display are not required to be provided additionally, thus it canbe prevented that the aperture ratio is reduced.

Embodiment Mode 2

In this embodiment mode, a pixel configuration different than that ofthe Embodiment Mode 1 is described.

As shown in FIG. 2, the pixel configuration of this embodiment mode isdifferent than Embodiment Mode 1 in that a driving transistor 25, and afirst control switch (SW20 a) and a second control switch (SW20 b)connected to one electrode of the driving transistor 25 are provided.For example, the first power source line 16 functions as an anode linefor the high gray scale display while the second power source line 17functions as an anode line for the low gray scale display. The otherparts of the pixel configuration are similar to those of Embodiment Mode1, therefore, the description thereof is omitted. In this embodimentmode also, the capacitor line 19 is not necessarily provided. Forexample, when the first power source line or the second power sourceline has a constant potential, one of them can be used as a capacitorline.

In such a pixel configuration, a signal line for the high gray scaledisplay and a signal line for the low gray scale display are notrequired to be provided, thus it can be prevented that the apertureratio is reduced.

Next, an operation thereof is described.

In this embodiment mode, a video signal voltage inputted to the signalline is determined similarly to Embodiment Mode 1. Based on such a videosignal voltage, the first control switch (SW20 a) and the second controlswitch (SW20 b) are determined to be turned ON or OFF according to thehigh gray scale display or the low gray scale display. That is to say,the first control switch (SW20 a) and the second control switch (SW20 b)are switched over exclusively. As a result, it can be selected that thedriving transistor 25 is connected to the first power source line 16 orthe second power source line 17. By switching over this connection, agate-source voltage (|Vgs|) of the driving transistor is switched over.Then, a light emitting element emits light based on the video signal forthe low gray scale display or the high gray scale display. It is to benoted that the first and second control switches 20 a and 20 b canoperate by a line sequential scanning at the same frequency as the writeswitch 11.

For example, in the case where a video signal for the low gray scaledisplay is inputted, a current does not flow even when the drivingtransistor is connected to the first power source line 16 for the highgray scale display since |Vgs| of the driving transistor 25 is low. Whenthe driving transistor 25 is connected to the second power source line17 for the low gray scale display by switching over the control switch,current flows since |Vgs| is high, thus the light emitting element 14emits light. In this manner, the low gray scale display or the high grayscale display can be performed.

In the low gray scale display, by switching over the first controlswitch (SW20 a) and the second control switch (SW20 b), the lightemission period can be shortened. As a result, |Vgs| of the drivingtransistor 25 for the low gray scale display can be high, thus an effectof variations in threshold voltages can be reduced.

In this embodiment mode, switches such as the write switch 11 and thecontrol switch 20 a and 20 b can be formed of an analog switch and thelike. FIG. 5 shows a pixel circuit in which a thin film transistor (TFT)containing polycrystalline silicon is used for the switches and eachtransistor of a pixel.

The transistor 40 corresponding to the write switch 11 (also referred toas a switching transistor) and transistors 41 a and 41 b correspondingto the control switches 20 a and 20 b are provided. The transistor 40 isformed of an n-channel TFT while the transistors 41 a and 41 b areformed of p-channel TFTs. It is to be noted that the transistor 40 mayhave a multi-gate structure in which a plurality of gate electrodes areprovided for one semiconductor film, for example a double-gate structurein which two gate electrodes are provided. The gate of the transistor 40is connected to a first scan line 42 while gates of the transistors 41 aand 41 b are connected to a second scan line 43 a and a third scan line43 b respectively. It is to be noted that by changing the polarity ofthe transistors 41 a and 41 b, the second scan line 43 a and the thirdscan line 43 b can be shared. As a result, the aperture ratio of thepixel can be increased. The transistor 25 can be formed of a p-channelTFT.

It is to be noted that an operating region of a TFT can be divided intoa linear region and a saturation region. It is preferable that thetransistors 40, 41 a and 41 b that function as switches operate in thelinear region while the transistor 25 for driving the light emittingelement 14 operates in the saturation region.

For the transistor, an amorphous silicon thin film transistor or otherthin film transistors can be used as well as a polycrystalline siliconthin film transistor. In other words, the structure of a transistor isnot limited in this embodiment mode.

In such a pixel, a timing at which the transistors 41 a and 41 bcorresponding to the control switches 20 a and 20 b are turned ON or OFFis determined. For example, it can be determined so that the transistor41 a is turned ON in 95% of one frame period while the transistor 41 bis turned ON in the rest 5% thereof. As a result, the light emissionperiod can be shortened in the low gray scale display. In other words,even in the case of increasing a gate-source voltage (|Vgs|) of thesecond driving transistor, a low gray scale display at a predeterminedluminance can be performed.

The control switch and the write switch can operate at the same drivingfrequency and they may also operate by a line sequential scanning aswell. Accordingly, the driving, that is the operation of the pixel inthis embodiment mode can be simplified.

FIG. 11 is a timing chart of the transistors 40, 41 a and 41 b. A Highsignal or a Low signal is inputted to the transistors 40, 41 a and 41 bin one frame period. In this embodiment mode, the description is made onthe case where the transistors 41 a and 41 b are p-channel transistors.A High signal is inputted to the transistor 41 a and a Low signal isinputted to the transistors 41 b at the same time. In the case of usingthe transistors 41 a and 41 b as shown in FIG. 5, a High or Low signalis to be inputted thereto at the same time. In this embodiment mode, aHigh signal is inputted to the transistor 41 a and a Low signal isinputted to the transistor 41 b in 30% of one frame period. Accordingly,the light emission period in the low gray scale display can be 30% ofone frame period. By setting the period in which a High signal isinputted to the transistors 41 a and 41 b to be as long as an integralmultiple of the period in which a High signal is inputted to thetransistor 40, they can operate at the same driving frequency.

The timing at which a High signal or a Low signal is inputted to thesetransistors is not limited to FIG. 11. For example, a High signal to thetransistor 41 a and a Low signal to the transistor 41 b may be inputtedat the same time as the transistor 40. This is because drivers for thetransistors 40, 41 a and 41 b are provided respectively. The period inwhich a High signal is inputted to the transistor 41 a is not limited toFIG. 11. In the case of performing a low gray scale display, forexample, a period in which the transistors 41 a and 41 b are turned ONcan be determined according to the gate-source voltage (|Vgs|) of thedriving transistor 25.

Heretofore described is the case of dividing the gray scales into thehigh gray scale display and the low gray scale display, however, theymay be divided into three or more groups as well. For example, byproviding first to third power source lines as anode lines for the highgray scale display, the middle gray scale display, and the low grayscale display and providing control switches connected to each of them,the gray scales can be displayed as the high gray scale display, themiddle gray scale display, and the low gray scale display.

Embodiment Mode 3

In this embodiment mode, a pixel configuration different than that ofEmbodiment Modes 1 and 2 is described.

As shown in FIG. 3, a pixel configuration of this embodiment mode isdifferent than that of Embodiment Mode 2 in that a first control switch(SW20 c) is provided between the capacitor 18 and a power source line26, a second control switch (SW20 d) is provided between the capacitor18 and the capacitor line 19, and the transistor 25 is connected to thepower source line 26. For example, the power source line 26 functions asan anode line for the high gray scale display while the capacitor line19 functions as an anode line for the low gray scale display. The otherparts of the pixel configuration are similar to those in Embodiment 2,therefore, the description thereof is omitted here.

According to the pixel configuration of this embodiment mode, theaperture ratio can be increased since the number of power source linesis small. Further, this embodiment mode is similar to Embodiment Modes 1and 2 in that a signal line for the high gray scale display and a signalline for the low gray scale display are not required to be providedadditionally.

A video signal potential in such a pixel configuration is determined asdescribed below. A video signal potential for the high gray scaledisplay is determined so that (a video signal potential for the highgray scale display)=(a potential of an anode line for the high grayscale display−|Vth| of a driving transistor) is satisfied, similarly toEmbodiment Mode 1.

A video signal potential for the low gray scale display is determined sothat (a potential of a capacitor line−|Vth| of a driving transistor)<(apotential of a power source line−|Vth| of the driving transistor)<(avideo signal potential for the low gray scale display) is satisfied.This is called a condition 1. Alternatively, (the potential of the powersource line−|Vth| of the driving transistor)<(the video signal potentialfor the low gray scale display)<(the potential of the capacitorline−|Vth| of the driving transistor) is to be satisfied. This is calleda condition 2. Further, (the video signal potential for the low grayscale display)−{(the potential of the power source line)−(the potentialof the capacitor line)}<(the potential of the power source line−|Vth| ofthe driving transistor) is satisfied. This is called a condition 3. Sucha video signal potential for the low gray scale display is determined byturning ON either the first control switch (SW20 c) or the secondcontrol switch (SW20 d) when writing the video signal.

In the low gray scale display, the light emission period can beshortened by the control switches 20 c and 20 d. At the same time, bychanging the control switches 20 c and 20 d by a potential differencebetween the capacitor line 19 and the power source line 26, |Vgs| of thedriving transistor 25 can be increased, thus an effect of variations inthreshold voltages can be reduced. It is to be noted that the first andthe second control switches 20 c and 20 d can operate by a linesequential scanning at the same frequency as the write switch 11.

In this embodiment mode, switches such as the write switch 11 and thecontrol switches 20 c and 20 d can be formed of an analog switch and thelike. FIG. 6 shows a pixel circuit in which a thin film transistor (TFT)containing polycrystalline silicon is used for the switches and eachtransistor of a pixel.

The transistor 40 corresponding to the write switch 11 (also referred toas a switching transistor) and the transistors 41 a and 41 bcorresponding to the control switches 20 c and 20 d are provided. Thetransistor 40 is formed of an n-channel TFT while the transistors 41 aand 41 b are formed of p-channel TFTs. It is to be noted that thetransistor 40 may have a multi-gate structure in which a plurality ofgate electrodes are provided for one semiconductor film, for example adouble-gate structure in which two gate electrodes are provided. Thegate of the transistor 40 is connected to the first scan line 42 whilethe gates of the transistors 41 a and 41 b are connected to the secondscan line 43 c and a third scan line 43 d respectively. It is to benoted that by changing the polarity of the transistors 41 a and 41 b,the second scan line 43 c and the third scan line 43 d can be shared. Asa result, the aperture ratio of the pixel can be increased. Thetransistor 25 can be formed of a p-channel TFT.

It is to be noted that an operating region of a TFT can be divided intoa linear region and a saturation region. It is preferable that thetransistors 40, 41 a and 41 b that function as switches operate in thelinear region while the transistor 25 for driving the light emittingelement 14 operates in the saturation region.

For the transistor, an amorphous silicon thin film transistor or otherthin film transistors can be used as well as a polycrystalline siliconthin film transistor. In other words, the structure of a transistor isnot limited in this embodiment mode.

In such a pixel, a timing at which the transistors 41 a and 41 bcorresponding to the control switches 20 c and 20 d are turned ON or OFFis determined. For example, the transistor 41 a is turned ON in 95% ofone frame period while the transistor 41 b is turned ON in the rest 5%thereof. On the contrary, the transistor 41 b may be turned ON in 95% ofone frame period while the transistor 41 a is turned ON in the rest 5%thereof. As a result, the light emission period can be shortened in thelow gray scale display. In other words, even in the case of increasing agate-source voltage (|Vgs|) of the second driving transistor, a low grayscale display at a predetermined luminance can be performed.

The control switch and the write switch can operate at the same drivingfrequency and they may also operate by a line sequential scanning aswell. Accordingly, the driving, that is the operation of the pixel inthis embodiment mode can be simplified.

FIGS. 12A and 12B are timing charts of the transistors 40, 41 c and 41 din the conditions 1 and 2 respectively. A High signal or a Low signal isinputted to the transistors 40, 41 c and 41 d in one frame period inFIGS. 12A and 12B. FIGS. 12A and 12B are similar to each other in that aLow signal is inputted to the transistor 41 c or 41 d connected to ahigh potential side of the power source line 26 and the capacitor line19 when writing a video signal. That is to say, a High or Low signal isto be inputted to the transistor 41 c or 41 d according to the condition1 or 2. In this embodiment mode, a period in which a Low signal isinputted to the transistor 41 c and a period in which a High signal isinputted to the transistor 41 d are 30% of one frame period in thecondition 1. In the condition 2, a period in which a High signal isinputted to the transistor 41 c and a period in which a Low signal isinputted to the transistor 41 d are 30% of one frame period. Then, alight emission period in the low gray scale display can be set 30%. Bydetermining a period in which a High signal is inputted to thetransistor 41 c so as to be as long as an integral multiple of a periodin which a High signal is inputted to the transistor 40, they canoperate at the same frequency.

The timing at which a High signal or a Low signal is inputted to theseswitches is not limited to FIGS. 12A and 12B. For example, a High signalmay be inputted to the transistor 41 c and a Low signal may be inputtedto the transistor 41 d at the same time as the transistor 40. However,it is not easy to switch over a signal to the transistors 41 c and 41 dwhen the transistor 40 is at High. This is because the transistors 41 cand 41 d are provided between the capacitor 18 and the power source line26, and between the capacitor 18 and the capacitor line 19 respectively.Specifically, when accumulating a charge into the capacitor 18 based ona video signal voltage when the transistor 40 is selected, thetransistors 41 c and 41 d are required to be fixed ON or OFF. Moreover,a period in which a High signal is inputted to the transistors 41 c and41 d is not limited to FIGS. 12A and 12B. In the case of the low grayscale display, for example, a period in which transistors 41 c and 41 dare turned ON can be determined according to the gate-source voltage(|Vgs|) of the driving transistor 25.

Heretofore described is the case of dividing the gray scales into thehigh gray scale display and the low gray scale display, however, theymay be divided into three or more groups as well. For example, byproviding first and second capacitor lines and control switchesconnected to each of them, the gray scales can be displayed as the highgray scale display, the middle gray scale display, and the low grayscale display.

Embodiment Mode 4

In this embodiment mode, a pixel configuration having a light emittingelement is described. It is to be noted that a thin film transistor(TFT) containing polycrystalline silicon is used as a transistor in thisembodiment mode.

As shown in FIG. 7A, a p-channel driving TFT (TFT is employed as atransistor) 301 provided over a substrate 300 having an insulatingsurface can have a crystalline silicon film as a semiconductor film. Acrystalline semiconductor film can be formed by thermal crystallizationor by laser irradiation, or using a catalytic effect of a metal elementsuch as nickel and titanium. In the case of irradiating laser, laserlight from a continuous oscillation laser (CW laser) or a pulsedoscillation laser (a pulsed laser) can be used. For the laser light, oneor a plurality of an Ar laser, a Kr laser, an excimer laser, a YAGlaser, a Y₂O₃ laser, a YVO₄ laser, a YLF laser, a YAlO₃ laser, a glasslaser, a ruby laser, an alexandrite laser, a Ti: sapphire laser, acopper vapor laser, or a gold vapor laser can be used. By irradiating afundamental wave of such laser light, and second to fourth harmonicwaves of the fundamental wave, large grain crystals can be formed. Forexample, a second harmonic (532 nm) and a third harmonic (355 nm) of aNd:YVO₄ laser (fundamental wave of 1064 nm) can be used.

A gate electrode and a gate line are provided over the semiconductorfilm with a gate insulating film interposed therebetween. Thesemiconductor film under the gate electrode corresponds to a channelforming region. The semiconductor film includes an impurity regioncorresponding to a source region or a drain region. The impurity regioncan be formed in a self-aligned manner by adding impurity elements suchas boron to the semiconductor film with the gate electrode as a mask. Afirst insulating film 316 is provided so as to cover the gate electrode,and contact holes are provided in the first insulating film over theimpurity region of the semiconductor film. Wirings are provided in thecontact holes, which function as a source wiring and a drain wiring. Theinsulating film can be formed of an organic material and an inorganicmaterial. For the organic material, a photosensitive ornon-photosensitive organic material (an organic resin material such aspolyimide, acrylic, polyamide, polyimide amide, benzocyclobutene, andsiloxane) and a resist can be used. Siloxane is a material which iscomposed of a skeleton formed by the bond of silicon and oxygen andwhich includes either an organic group containing at least hydrogen(such as an alkyl group or aromatic hydrocarbon) or a fluoro group orboth as a substituent. Alternatively, a fluoro group may be used as thesubstituent. For the inorganic material, an insulating film containingoxygen or nitrogen, such as a silicon oxide (SiO_(x)), silicon nitride(SiN_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y), silicon nitrideoxide (SiN_(x)O_(y))(x>y)(x, y=1, 2 . . . ) and polysilazane, which isformed from a liquid material including a polymer material having a bondof silicon (Si) and nitrogen (N), can be used. For the insulating film,a stacked-layer structure of these films may be used as well. For theinsulating film, in particular, it is preferable to form an organicmaterial for planarizing the film and then an inorganic materialthereover for preventing moisture and oxygen from being absorbed in theorganic material.

A first electrode 311 of a light emitting element is provided so as tobe electrically connected to a drain electrode. The first electrode 311functions as an anode of the light emitting element. A second insulatingfilm is provided so as to cover the first electrode 311. The secondinsulating film 317 has an aperture on the first electrode. Anelectroluminescent layer 312 is provided in the aperture and a secondelectrode 313 of the light emitting element is provided so as to coverthe electroluminescent layer and the second insulating film. The secondelectrode functions as a cathode of the light emitting element. That is,the light emitting element includes the first electrode 311, theelectroluminescent layer 312, and the second electrode 313.

The electroluminescent layer 312 includes an HIL, (hole injectionlayer), an HTL (hole transporting layer), an EML (light emitting layer),an ETL (electron transporting layer), and an ELL (electron injectionlayer) stacked in this order from the first electrode 311 side.Typically, CuPu is used for the HIL, a-NPD is used for the HTL, BCP isused for the ETL, and BCP:Li is used for the EIL.

For the electroluminescent layer 312, materials which exhibit red (R),green (G), and blue (B) color light emissions can be used. As a result,a full color display can be performed. Such materials which exhibit red(R), green (G), and blue (B) color light emissions may be selectivelyformed by an evaporation method using an evaporation mask, a dropletdischarging method (also referred to as an ink-jetting method) and thelike. Specifically, CuPu or PEDOT is used for the HIL, a-NPD is used forthe HTL, BCP or Alq₃ is used for the ETL, and BCP:Li or CaF₂ is used forthe EIL. For example, Alq₃ doped with a dopant (DCM and the like for Rand DMQD and the like for G) corresponding to each light emission colorof R, C, and B may be used for the EML.

The structure of the electroluminescent layer 312 is not limited to theaforementioned stacked-layer structure. For example, theelectroluminescent layer 312 may have any of a single layer structure, astacked-layer structure, and a mixed-layer structure having nointerfaces between the layers. Moreover, a singlet material, a tripletmaterial, or a mixed material of them can be used. For example, atriplet material is used as a material which exhibits red color (R)light emission while a single material can be used as a material whichexhibits green color (G) and blue color (B). Furthermore, any of anorganic material containing a low polymeric material, a high polymericmaterial, and a middle polymeric material, an inorganic materialtypified by molybdenum oxide superior in electron injection property, amixed material of organic and inorganic materials may be used.

In the case of forming an electroluminescent layer which exhibits whitecolor light emission, a color filter, or a color filter and a colorconversion layer, and the like may be additionally provided. As aresult, a full color display can be performed. This color filter and thecolor conversion layer can be formed over a second substrate 315 whichis to be attached to the first substrate thereafter.

The material of the first electrode 311 and the second electrode 313 areselected in consideration of the work function.

For example, the first electrode 311 which functions as an anode ispreferably formed of a metal, an alloy, a conductive compound, and amixture of them, each having a large work function (work function of 4.0eV or higher). As specific materials, ITO (indium tin oxide), IZO(indium zinc oxide) obtained by mixing 2 to 20% of zinc oxide (ZnO) intoindium oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd), or nitride of a metal material (TiN) and the like can beused.

On the other hand, the second electrode 313 which functions as a cathodeis preferably formed of a metal, an alloy, a conductive compound, and amixture of them, each having a low work function (a work function of 3.8eV or lower). As specific materials, there are elements belonging togroup 1 or 2 of Periodic Table of Elements, that are an alkaline metalsuch as Li and Cs, Mg, Ca, and Sr, an alloy containing these (Mg:Ag,Al:Li), a compound (LiF, CsF, CaF₂), and a transition metal containing arare earth metal. Moreover, when the second electrode is required totransmit light, the aforementioned metal or alloy containing them may beformed quite thin and stacked to a metal such as ITO (including analloy).

The first electrode 311 and the second electrode 313 can be formed by anevaporation method, a sputtering method, an ink-jetting method and thelike.

The first electrode 311 and the second electrode 313 can both be ananode or a cathode according to the pixel configuration. For example, inthe case of forming the driving TFT as an n-channel TFT, the firstelectrode 311 functions as a cathode while the second electrode 313functions as an anode. In the case of forming the driving TFT as ann-channel TFT, other TFTs in the pixel can be formed of n-channel TFTs.As a result, a TFT manufacturing process can be simplified.

It is preferable to provide a passivation film 314 containing nitrogenover the second electrode 313. For example, a silicon nitride film ispreferable. A diamond-like carbon (DLC) film may be provided as thepassivation film as well. The passivation film 314 can be formed by asputtering method and a CVD method. The passivation film 314 can preventmoisture and oxygen that cause degradation from entering.

The space between the first substrate 300 and the second substrate(sealing substrate) 315 attached to each other may be injected withnitrogen, and a drying agent may be additionally provided therein.Instead of injecting nitrogen, the space may be filled with a resinwhich transmits light and has high water absorbing property as well. Asa result, oxygen and moisture can be prevented from entering.

After attaching the first substrate 300 and the second substrate 315 toeach other, the edges of these substrates may be covered with the firstelectrode 311, the second electrode 313, other electrodes, and aninsulating film. As a result, oxygen and moisture can be prevented fromentering.

In order to enhance the contrast, a polarizer or a circular polarizermay be provided in a display region of the first substrate 300 or adisplay region of the second substrate 315. As a result, quality of theblack display can be enhanced and the contrast can be improved.

A display device having such a pixel configuration emits light based ona video signal inputted from the signal line. Specifically, the videosignal is an analog signal having a voltage value, as described above.When the analog signal is inputted when a switching TFT is ON, a drivingTFT is turned ON and the light emitting element emits light based on thegate-source voltage (Vgs) of the driving TFT.

For example, in a display device having the first electrode 311 and thesecond electrode 313 formed of a material which transmits light, lightis emitted in both directions (directions of arrows) relatively to thelight emitting element as shown in FIG. 7A. Such a display device cantransmit light when no light is being emitted.

In a display device having the second electrode 313 formed of a materialwhich transmits light, light is emitted only to the sealing substrate315 side as shown in FIG. 7B. Therefore, the first electrode 311 isformed of a material which does not transmit light. Furthermore, it ispreferable to form the first electrode 311 of a highly reflectivematerial. The other parts are similar to FIG. 7A, therefore, thedescription is omitted here. In the case where the aperture ratio may bereduced due to the transistors in the pixel, it is preferable that thelight be emitted to the sealing substrate 315 side as shown.

In a display device having the first electrode 311 formed of a materialwhich transmit light, light is emitted only to the first substrate 300side as shown in FIG. 7C. Therefore, the second electrode 313 is formedof a material which does not transmit light. Furthermore, it ispreferable to form the second electrode 313 of a highly reflectivematerial. The other parts are similar to FIG. 7A, therefore, thedescription is omitted here.

As shown in FIGS. 7B and 7C, by using a highly reflective conductivefilm for the electrode of the light emitting element provided on a sideto which the light is not emitted, light can be effectively utilised.

In this embodiment mode, the electrode which transmits light is formedby forming a conductive film which does not transmit light so as to bethin enough to transmit light and stacking thereover a conductive filmwhich transmits light, for example an ITO.

Embodiment Mode 5

A display device having the pixel configuration described in theaforementioned embodiment modes can be applied to various electronicapparatuses. The electronic apparatuses include a portable informationterminal (a portable phone, a mobile computer, a portable game machine,an electronic book and the like), a video camera, a digital camera, agoggle type display, a display, a navigation system, and the like.Specific examples of these electronic apparatuses are shown in FIGS. 8Ato 8E.

FIG. 8A illustrates a display including a housing 4001, an audio outputportion 4002, a display portion 4003 and the like. The display devicehaving the pixel configuration of the invention can be applied to thedisplay portion 4003. As a result, a display with less effect ofvariations of driving transistors can be performed even in the low grayscale display. It is to be noted that the display device includes alldisplay devices for displaying information such as for a computer, TVbroadcasting receiver, and advertising display.

FIG. 8B illustrates a mobile computer including a main body 4101, astylus 4102, a display portion 4103, operating buttons 4104, an externalinterface 4105 and the like. The display device having the pixelconfiguration of the invention can be applied to the display portion4103. As a result, a display with less effect of variations of drivingtransistors can be performed even in the low gray scale display.

FIG. 8C illustrates a game machine including a main body 4201, a displayportion 4202, operating buttons 4203, and the like. The display devicehaving the pixel configuration of the invention can be applied to thedisplay portion 4202. As a result, a display with less effect ofvariations of driving transistors can be performed even in the low grayscale display.

FIG. 8D illustrates a portable phone including a main body 4301, anaudio output portion 4302, an audio input portion 4303, a displayportion 4304, an operating switch 4305, an antenna 4306, and the like.The display device having the pixel configuration of the invention canbe applied to the display portion 4304. As a result, a display with lesseffect of variations of driving transistors can be performed even in thelow gray scale display.

FIG. 8E illustrates an electronic book reader including a displayportion 4401 and the like. The display device having the pixelconfiguration of the invention can be applied to the display portion4401. As a result, a display with less effect of variations of drivingtransistors can be performed even in the low gray scale display.

As described above, an application range of the invention is quite wideand the invention can be applied to electronic apparatuses of variousfields. By applying the invention, a display with less effect ofvariations of driving transistors can be performed even in the low grayscale display. Moreover, by using a flexible substrate for an insulatingsubstrate of an active matrix substrate, a thin and lightweight devicecan be realized.

Next, results of the evaluation of variations of driving TFTs in thecase of 64-gray scale (6-bit) display are shown.

In the highest luminance 63 of the high gray scale display, the currentis set 0.56 μA (200 cd/m²) and a drain-source voltage (Vds) of thedriving TFT is set −15 V. Table 1 shows values of L/W when the length ofthe channel length of the driving TFT of each sample is L and the length(width) perpendicular to the channel length of the driving TFT of eachsample is W.

TABLE 1 sample 1 sample 2 sample 3 sample 4 L/W 80/4 120/4 320/4 480/4

FIG. 9 shows variations of currents (Ids) flowing through the drivingTFTs when a 64 gray scale (6-bit) display is performed by inputtingvideo signal voltage to the signal line under the aforementionedconditions. The variations in the currents (Ids) flowing through thedriving TFTs at this time present at a ratio of 3 s. That is to say, thevariations are obtained by the formula: variations=Ids (3 s)/Ids(average).

As shown in FIG. 9, the variations in the current (Ids) flowing throughthe driving TFT are increased in the low gray scale display. Accordingto this experiment, it can be found out that the variations in drivingTFTs are notable in the case of the low gray scale display in which avery small amount of current is supplied to the light emitting element.The inventors found out from this experiment that the variations in thethreshold voltages of the driving TFTs are notable in the case of thelow gray scale display.

This application is based on Japanese Patent Application serial no.2004-134723 filed in Japan Patent Office on Apr. 28, 2004 the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device including a circuitimplementing a circuit diagram comprising: a first line; a second line;a first transistor comprising a gate insulating layer; a secondtransistor comprising a gate insulating layer; a third transistorcomprising a gate insulating layer; a fourth transistor comprising agate insulating layer; a capacitor; and a light emitting element,wherein, in the circuit diagram, a gate of the first transistor is indirect contact with one of a source and a drain of the fourthtransistor, wherein, in the circuit diagram, the gate of the firsttransistor is in indirect contact with one of a source and a drain ofthe second transistor through the capacitor, wherein, in the circuitdiagram, one of a source and a drain of the first transistor is inelectrical contact with the light emitting element, wherein, in thecircuit diagram, a first terminal of the capacitor is in direct contactwith the gate of the first transistor, wherein, in the circuit diagram,a second terminal of the capacitor is in electrical contact with thefirst line through the second transistor, wherein, in the circuitdiagram, the second terminal of the capacitor is in electrical contactwith the second line through the third transistor, wherein, in thecircuit diagram, the second terminal of the capacitor is in directcontact with the one of the source and the drain of the secondtransistor, and wherein, in the circuit diagram, the second terminal ofthe capacitor is in direct contact with one of a source and a drain ofthe third transistor.
 2. The semiconductor device according to claim 1,wherein the first transistor is a driving transistor.
 3. Thesemiconductor device according to claim 1, wherein the first line andthe second line are power source lines.
 4. The semiconductor deviceaccording to claim 1, wherein a potential of the second line is higherthan a potential of the first line.
 5. The semiconductor deviceaccording to claim 1, wherein the first transistor is an amorphoussilicon thin film transistor or a polycrystalline silicon thin filmtransistor.
 6. The semiconductor device according to claim 1, whereinthe first line is a line for a high gray scale display and the secondline is a line for a low gray scale display.
 7. A display devicecomprising the semiconductor device according to claim
 1. 8. Anelectronic device comprising the semiconductor device according toclaim
 1. 9. A semiconductor device including a circuit implementing acircuit diagram comprising: a first line; a second line; a third line; afirst transistor comprising a gate insulating layer; a second transistorcomprising a gate insulating layer; a third transistor comprising a gateinsulating layer; a fourth transistor comprising a gate insulatinglayer; a capacitor; and a light emitting element, wherein, in thecircuit diagram, a gate of the first transistor is in direct contactwith one of a source and a drain of the fourth transistor, wherein, inthe circuit diagram, the gate of the first transistor is in indirectcontact with one of a source and a drain of the second transistorthrough the capacitor, wherein, in the circuit diagram, one of a sourceand a drain of the first transistor is in electrical contact with thelight emitting element, wherein, in the circuit diagram, a firstterminal of the capacitor is in direct contact with the gate of thefirst transistor, wherein, in the circuit diagram, a second terminal ofthe capacitor is in electrical contact with the first line through thesecond transistor, wherein, in the circuit diagram, the second terminalof the capacitor is in electrical contact with the second line throughthe third transistor, wherein, in the circuit diagram, the secondterminal of the capacitor is in direct contact with the one of thesource and the drain of the second transistor, wherein, in the circuitdiagram, the second terminal of the capacitor is in direct contact withone of a source and a drain of the third transistor, wherein, in thecircuit diagram, a gate of the second transistor is directly connectedto the third line, and wherein the first line is a power source line.10. The semiconductor device according to claim 9, wherein the firsttransistor is a driving transistor.
 11. The semiconductor deviceaccording to claim 9, wherein the second line is a power source line.12. The semiconductor device according to claim 9, wherein a potentialof the second line is higher than a potential of the first line.
 13. Thesemiconductor device according to claim 9, wherein the first transistoris an amorphous silicon thin film transistor or a polycrystallinesilicon thin film transistor.
 14. The semiconductor device according toclaim 9, wherein the first line is a line for a high gray scale displayand the second line is a line for a low gray scale display.
 15. Adisplay device comprising the semiconductor device according to claim 9.16. An electronic device comprising the semiconductor device accordingto claim 9.